Method of abnormality detection of oxygen concentration sensor

ABSTRACT

Abnormalities are detected in an oxygen concentration sensor employed to sense the oxygen concentration in engine exhaust gas for obtaining data to control the air/fuel ratio of a fuel mixture supplied to the engine, in which the oxygen concentration sensor includes a sensor cell element and an oxygen pump element. The method functions to detect an abnormality of the oxygen concentration sensor on the basis of the value of sensor voltage which is developed by the sensor cell element and at least one of a set of parameters consisting of an engine operating condition, the value of pump current which flows between the electrodes of the oxygen pump element, and an air/fuel ratio compensation value which is derived from a sensed oxygen concentration level.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of abnormality detection foran oxygen concentration sensor which senses the level of oxygenconcentration in a gas such as engine exhaust gas.

2. Description of Background Information

In order to reduce exhaust gas pollutants and to improve the fuelconsumption of an internal combustion engine, it is now common practiceto employ an oxygen concentration sensor to detect the concentration ofoxygen in the engine exhaust gas, and to execute feedback control of theair/fuel ratio of the mixture supplied to the engine such as to hold theair/fuel ratio at a target value. This feedback control is performed inaccordance with an output signal from the oxygen concentration sensor.

One form of oxygen concentration sensor can be employed for suchair/fuel ratio control functions by producing an output signal whichvaries in level in accordance with the oxygen concentration in theengine exhaust gas. Such an oxygen concentration sensor has beendisclosed for example in Japanese patent laid-open No. 52-72286. Thissensor consists of an oxygen ion-conductive solid electrolytic memberformed as a flat plate having a pair of electrodes formed on two mainfaces, with one of these electrode faces forming part of a gas holdingchamber. The gas holding chamber communicates with a gas which is to bemeasured, i.e. exhaust gas, through a lead-in aperture. With such anoxygen concentration sensor, the oxygen ion-conductive solidelectrolytic member and its electrodes function as an oxygen pumpelement. By passing a flow of current between the electrodes such thatthe electrode within the gas holding chamber becomes a negativeelectrode, oxygen gas within the gas holding chamber adjacent to thisnegative electrode becomes ionized, and flows through the solidelectrolytic member towards the positive electrode, to be therebyemitted from that face of the sensor element as gaseous oxygen. Thecurrent flow between the electrodes is a boundary current value which issubstantially constant, i.e. is substantially unaffected by variationsin the applied voltage, and is proportional to the oxygen concentrationwithin the gas under measurement. Thus, by sensing the level of thisboundary current, it is possible to measure the oxygen concentrationwithin the gas which is under measurement. However, if such an oxygenconcentration sensor is used to control the air/fuel ratio of themixture supplied to an internal combustion engine, by measuring theoxygen concentration within the engine exhaust gas, it will only bepossible to control the air/fuel ratio to a value which is in the leanregion, relative to the stoichiometric air/fuel ratio. It is notpossible to perform air/fuel ratio control to maintain a target air/fuelratio which is set in the rich region. An oxygen concentration sensorwhich will provide an output signal level varying in proportion to theoxygen concentration in engine exhaust gas for both the lean region andthe rich region of the air/fuel ratio has been proposed in Japanesepatent laid-open No. 59-192955. This sensor consists of two oxygenion-conductive solid electrolytic members each formed as a flat plate,and each provided with a pair of electrodes. Two opposing electrodefaces, i.e. one face of each of the solid electrolytic members, formpart of a gas holding chamber which communicates with a gas undermeasurement, via a lead-in aperture. The other electrode of one of thesolid electrolytic members faces into the atmosphere. In this oxygenconcentration sensor, one of the solid electrolytic members and a pairof electrodes functions as an oxygen concentration ratio sensor cellelement. The other solid electrolytic member and a pair of electrodesfunction as an oxygen pump element. If the voltage which is generatedbetween the electrodes of the oxygen concentration ratio sensor cellelement is higher than a reference voltage value, then current issupplied between the electrodes of the oxygen pump element such thatoxygen ions flow through the oxygen pump element towards the electrodeof that element which is within the gas holding chamber. If the voltagedeveloped between the electrodes of the sensor cell element is lowerthan the reference voltage value, then a current is supplied between theelectrodes of the oxygen pump element such that oxygen ions flow throughthat element towards the oxygen pump element electrode which is on theopposite side to the gas holding chamber. In this way, a value ofcurrent flow between the electrodes of the oxygen pump element isobtained which varies in proportion to the oxygen concentration of thegas under measurement, both in the rich and the lean regions of theair/fuel ratio.

However, with such an oxygen concentration sensor if an abnormality ofthe sensor should occur, then not only will it be impossible to achievea desired oxygen concentration sensing characteristic, but it will be nolonger possible to accurately control the air/fuel ratio, so that theeffectiveness of exhaust gas pollution removal will be reduced. It istherefore desirable to provide a method of reliably detecting anyabnormality of components of such an oxygen concentration sensor.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide a method ofabnormality detection for an oxygen concentration sensor whereby anabnormality of oxygen concentration sensing can be reliably detected.

A method of abnormality detection for an oxygen concentration sensoraccording to the present invention is characterized in that anabnormality of oxygen concentration sensing is detected based upon thevalue of sensor voltage developed between the electrodes of a sensorcell element of the oxygen concentration sensor and at least one of thefollowing:

an engine operating condition;

the value of pump current which flows between the electrodes of anoxygen pump element;

an air/fuel ratio compensation value which is utilized to control theair/fuel ratio of a mixture supplied to an engine such as to maintainthat ratio at a target air/fuel ratio.

More specifically, the present invention provides a method ofabnormality detection for an oxygen concentration sensor of an air/fuelratio control apparatus of an internal combustion engine. The sensorincludes first and second pairs of electrodes disposed mutually opposingwith each of the pairs each sandwiching an oxygen ion-conductive solidelectrolytic member. A gas diffusion control means supplies an exhaustgas of an internal combustion engine to the vicinity of one electrode ofeach of the first and second pairs of electrodes. Voltage applying meansapply a pump voltage, determined in accordance with a voltage differencebetween a sensor voltage which is developed between the first pair ofelectrodes and a reference voltage between the second pair of electrodesto maintain the sensor voltage at the reference voltage. The voltageapplying means thereby produces as an output which represents an oxygenconcentration sensing value, a value of pump current which flows betweenthe second pair of electrodes, means for deriving an air/fuel ratiocompensation value in accordance with the oxygen concentration sensingvalue for controlling an air/fuel ratio of a mixture supplied to theinternal combustion engine such as to maintain the air/fuel ratio at atarget air/fuel ratio which is determined in accordance with engineoperating conditions, and drive means for driving an air/fuel ratiocontrol means of the internal combustion engine in accordance with acorrected air/fuel ratio control value which is obtained by applyingcompensation to an air/fuel ratio control value in accordance with theair/fuel ratio compensation value. The method of the present inventioncharacterized in that an abnormality of the oxygen concentration sensoris detected based upon the value of sensor voltage developed between thefirst pair of electrodes and at least one of the following:

an engine operating condition;

the value of pump current which flows between the second pair ofelectrodes and

the air/fuel ratio compensation value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an electronic control fuel injectionapparatus equipped with an oxygen concentration sensor, suitable forapplication of the abnormality detection method of the presentinvention;

FIG. 2 is a diagram for illustrating the internal configuration of anoxygen concentration sensor detection unit;

FIG. 3 is a block circuit diagram of the interior of an ECU (ElectronicControl Unit), and;

FIGS. 4a, 4b, 4ba and 4bb are flow charts for assistance in describingthe operation of a CPU.

DETAILED DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will now be described, referringto the drawings. FIGS. 1 through 3 show an electronic fuel controlapparatus for an internal combustion engine incorporating an oxygenconcentration sensor which utilizes the output compensation method ofthe present invention. In this apparatus, an oxygen concentration sensordetection unit 1 is mounted within an exhaust pipe 3 of an engine 2,upstream from a catalytic converter 5. Inputs and outputs of the oxygenconcentration sensor detection unit 1 are coupled to an ECU (ElectronicControl Unit) 4.

The protective case 11 of the oxygen concentration sensor detection unitcontains an oxygen ion-conductive solid electrolytic member 12 which mayhave, for example a substantially rectangular shape of the form shown inFIG. 2. A gas holding chamber 13 is formed in the interior of the solidelectrolytic member 12, and communicates via a lead-in aperture 14 withexhaust gas at the exterior of solid electrolytic member 12,constituting a gas to be sampled. The lead-in aperture 14 is positionedsuch that the exhaust gas will readily flow from the interior of theexhaust pipe into the gas holding chamber 13. In addition, anatmospheric reference chamber 15 is formed within the solid electrolyticmember 12, into which atmospheric air is led. The atmospheric referencechamber 15 is separated from the gas holding chamber 13 by a portion ofthe solid electrolytic member 12 serving as a partition. As shown, pairsof electrodes 17a, 17b and 16a, 16b are respectively formed on thepartition between chambers 13 and 15 and on both sides of the wall ofchamber 13 opposite to chamber 15. The solid electrolytic member 12functions in conjunction with the electrodes 16a and 16b as an oxygenpump element 18, and functions in conjunction with electrodes 17a and17b as a sensor cell element 19. A heater element 20 is mounted on theexternal surface of the atmospheric reference chamber 15.

The oxygen ion-conductive solid electrolytic member 12 can be formed,for example, of ZrO₂ (irconium dioxide), while the electrodes 16athrough 17b are each formed of platinum.

As shown in FIG. 3, ECU 4 includes an oxygen concentration sensorcontrol section, consisting of a differential amplifier 21, a referencevoltage source 22, and resistors 23 and 24. Electrode 16b of the oxygenpump element 18 and electrode 17b of sensor cell element 19 are eachconnected to ground potential, and electrode 17a of senso cell element19 is connected through resistor 24 to a supply voltage V_(cc).Electrode 17a is also connected to an input terminal of the differentialamplifier 21, which produces an output voltage in accordance with thedifference between a voltage developed across electrodes 17a and 17b ofthe sensor cell element 19 and the output voltage from the referencevoltage source 22. The output voltage from reference voltage source 22is a value corresponding to a stoichiometric air/fuel ratio (for example0.4 V). The output terminal of differential amplifier 21 is connectedthrough current sensing resistor 23 to electrode 16a of the oxygen pumpelement 18. The terminals of current sensing resistor 23 constitute apair of output terminals of the oxygen concentration sensor, and arecoupled to a microcomputer which constitutes the control circuit 25.

A throttle valve opening sensor 31 which produces an output voltage inaccordance with the degree of opening of throttle valve 26, which can beimplemented as a potentiometer, is coupled to control circuit 25.Control circuit 25 is also connected to an absolute pressure sensor 32which is mounted in the intake pipe 27 at a position downstream from thethrottle valve 26 and produces an output voltage varying in level inaccordance with the absolute pressure within the intake pipe 27. Controlcircuit 25 is also connected to a water temperature sensor 33 whichproduces an output voltage varying in level in accordance with thetemperature of the engine cooling water, and to a crank angle sensor 34which produces a signal consisting of successive pulses respectivelyproduced in synchronism with rotation of the crankshaft (not shown inthe drawings) of engine 2. Control circuit 25 is also connected to aninjector 35, provided in the intake pipe 27, near the intake valves (notshown in the drawings) of engine 2.

Control circuit 25 includes an analog/digital (A/D) converter 39 forconverting the voltage V_(S) developed between electrodes 17a and 17b ofsensor cell element 19 into a digital signal, and an A/D converter 40receives the voltage developed across the current sensing resistor 23 asa differential input and converts that voltage to a digital signal.Control circuit 25 also includes a level converter circuit 41 whichperforms level conversions of each of the output signals from thethrottle valve opening sensor 31, the absolute pressure sensor 32, andthe water temperature sensor 33. The resultant level-converted signalsfrom level converter circuit 41 are supplied to inputs of a multiplexer42. Control circuit 25 also includes an A/D converter 43 which convertsthe output signals from multiplexer 42 to a digital form, a waveformshaping circuit 44 which executes waveform shaping of the output signalfrom the crank angle sensor 34 to produce TDC (top dead center) signalpulses as output, and a counter 45 which counts a number of clock pulses(produced from a clock pulse generating circuit which is not shown inthe drawings) during each interval between successive TDC pulses fromthe waveform shaping circuit 44. control circuit 25 further includes adrive circuit 46a for driving the injector 35, a pattern display drivecircuit 46b for driving a pattern display unit 38, a CPU (centralprocessing unit) 47 for performing digital computation in accordancewith a program with a ROM (read-only memory) 48 having variousprocessing program and data stored therein, and a RAM (random accessmemory) 49. The A/D converters 39, 40 and 43, multiplexer 42, counter45, drive circuits 46a, 46 b, CPU 47, ROM 48 and RAM 49 are mutuallyinterconnected by an input/output bus 50. The TDC signal is suppliedfrom the waveform shaping circuit 44 to the CPU 47. The control circuit25 also includes a heater current supply circuit 51, which can forexample include a switching element which is responsive to a heatercurrent supply command from CPU 47 for applying a voltage between theterminals of heater element 20, to thereby supply heater current andproduce heating of heater element 20.

The voltage V_(S) developed between electrodes 17a and 17b of the sensorcell element 19 transferred from A/D converter 39, data representing apump current value I_(P) corresponding to the current flow through theoxygen pump element 18 transferred from A/D converter 40 together withdata representing a degree of throttle valve opening θ_(TH), datarepresenting the absolute pressure P_(BA) within the intake pipe, anddata representing the cooling water temperature T_(W) respectivelyselectively and transferred by A/D converter 43, are supplied to CPU 47over the I/O bus 50. In addition a count value from counter 45, which isattained during each period of the TDC pulses, is also supplied to CPU47 over I/O bus 50. The CPU 47 executes read-in of each of these data inaccordance with a processing program which is stored in the ROM 48, andcomputes a fuel injection time interval T_(OUT) for injector 35 on thebasis of the data, in accordance with a fuel injection quantity forengine 2 which is determined from predetermined equations. Thiscomputation is performed by means of a fuel supply routine, which isexecuted in synchronism with the TDC signal. The injector 35 is thenactuated by drive circuit 46a for the duration of the fuel injectiontime interval T_(OUT), to supply fuel to the engine.

The fuel injection time interval T_(OUT) can be obtained for examplefrom the following equation:

    T.sub.OUT =T.sub.I ×K.sub.O2 ×K.sub.WOT ×K.sub.TW (1)

In the above equation, T_(I) is the basic supply quantity, which isdetermined in accordance with the engine speed of rotation N_(e) and theabsolute pressure P_(BA) in the intake pipe and expresses a basicinjection time interval. K_(O2) is a feedback compensation coefficientfor the air/fuel ratio, which is set in accordance with the outputsignal level from the oxygen concentration sensor. K_(WOT) is a fuelquantity increment compensation coefficient, which is applied when theengine is operating under high load. K_(TW) is a cooling watertemperature coefficient. T_(I), K_(O2) K_(WO2) and K_(TW) arerespectively set by a subroutine of the fuel supply routine.

When the supply of pump current to the oxygen pump element begins, ifthe air/fuel ratio of the mixture which is supplied to engine 2 at thattime is in the lean region, then the voltage which is produced betweenelectrodes 17a and 17b of the sensor cell element 19 will be lower thanthe output voltage from the reference voltage source 22, and as a resultthe output voltage level from the differential amplifier 21 will bepositive. This positive voltage is applied through the series-connectedcombination of resistor 23 and oxygen pump element 18. A pump currentthereby flows from electrode 16a to electrode 16b of the oxygen pumpelement 18, so that the oxygen within the gas holding chamber 13 becomesionized by electrode 16b, and flows through the interior of oxygen pumpelement 18 from electrode 16b, to be ejected from electrode 16a asgaseous oxygen. Oxygen is thereby drawn out of the interior of the gasholding chamber 13.

As a result of this withdrawal of oxygen from the gas holding chamber13, a difference in oxygen concentration will arise between the exhaustgas within gas holding chamber 13 and the atmospheric air within theatmospheric reference chamber 15. A voltage V_(S) is thereby producedbetween electrodes 17a and 17b of the sensor cell element 19 at a leveldetermined by this difference in oxygen concentration, and the voltageV_(S) is applied to the inverting input terminal of differentialamplifier 21. The output voltage from differential amplifier 21 isproportional to the voltage difference between the voltage V_(S) and thevoltage produced from reference voltage source 22, hence the pumpcurrent is proportional to the oxygen concentration within the exhaustgas. The pump current value is output as a value of voltage appearingbetween the terminals of current sensing resistor 23.

When the air/fuel ratio is within the rich region, the voltage V_(S)will be higher than the output voltage from reference voltage source 22,hence the output voltage from differential amplifier 21 will be invertedfrom a positive to a negative level. In response to this negative levelof output voltage, the pump current which flows between electrodes 16aand 16b of the oxygen pump element 18 is reduced, and the direction ofcurrent flow is reversed. Thus, since the direction of the pump currentflow is now from the electrode 16b to electrode 16a, oxygen will beionized by electrode 16a, so that oxygen will be transferred as ionsthrough oxygen pump element 18 to electrode 16b, to be emitted asgaseous oxygen within the gas holding chamber 13. In this way, oxygen isdrawn into gas holding chamber 13. The supply of pump current is therebycontrolled such as to maintain the oxygen concentration within the gasholding chamber 13 at a constant value, by drawing oxygen into or out ofchamber 13, so that the pump current I_(P) will always be proportionalto the oxygen concentration in the exhaust gas, both for operation inthe lean region and in the rich region of the air/fuel ratio. The valueof the feedback compensation coefficient K_(O2) referred to above isestablished in accordance with the pump current value I_(P), in a K_(O2)computation subroutine. This subroutine can for example be similar to aprogram which is described in U.S. Pat. No. 4,566,419. Specifically, theoxygen concentration representing value VO₂, determined in accordancewith I_(P) is compared with a target air/fuel ratio V_(ref) (which isdetermined in accordance with the engine parameters), and if VO₂<V_(ref), the computation K_(O2) -Δ is executed, while if VO₂ >V_(ref),the computation K_(O2) +Δ is executed.

An operating sequence for the oxygen concentration sensor abnormalitydetection method of the present invention will now be described,referring to the operating flow of CPU 47 shown in FIGS. 4a and 4b. Thisabnormality detection function is provided as an abnormality detectionsubroutine of of the fuel supply routine, and therefore is executed eachtime the fuel supply routine is executed.

In the operating sequence, CPU 47 first judges whether or not activationof the oxygen concentration sensor has been completed (step 61). Thisdecision can be based for example upon whether or not a predeterminedtime duration has elapsed since the supply of heater current to theheater element 20 was initiated. If activation of the oxygenconcentration sensor has been completed, the voltage V_(S) betweenelectrodes 17a and 17b of sensor cell element 19 is read in, and adecision is made as to whether or not V_(S) is 0 [V] (step 62). If V_(S)≠0 [V], then a timer A (not shown in the drawings) in CPU 47 is reset,to begin counting up from zero. If on the other hand V_(S) =0 [V], thena decision is made as to whether or not a count value T_(A) of timer Ais greater than a time interval t_(O) (step 64). If T_(A) ≧t_(O), thenthe pump current value I_(P) is read in, and a decision is made as towhether or not I_(P) is higher than an upper limit value I_(PLH) (step65). If I_(P) >I_(PLH), then since an excessively high value of pumpcurrent is flowing while a condition of V_(S) =0 (V) is continuouslymaintained, this is taken as indicating a short-circuit betweenelectrodes 17a and 17b of the sensor cell element 19. A "sensor cellelement shortcircuit" display command is therefore issued to drivecircuit 46b (step 66). If I_(P) ≦I_(PLH), then a decision is made as towhether or not the value of the feedback compensation coefficientK_(O2), computed by a K_(O2) computation subroutine, is higher than anupper limit value K_(O2LH) (step 67). If K_(O2) >K_(O2LH), then thisindicates a condition in which V_(S) is continuously maintained at 0 (V)even though an excessively high value of pump current does not flow inthe positive direction, and the feedback compensation coefficient K_(O2)is excessively high. This condition is taken as indicating ashort-circuit between electrodes 17a and 17b of the sensor cell element19, and a "sensor cell element short-circuit" display command istherefore issued to drive circuit 46b (step 66).

After reset of timer A, if it is judged that the count value T_(A) oftimer A has not yet attained time t_(o), or if it is judged that K_(O2)≦K_(O2LH), then a decision is made as to whether or not voltage V_(S) isequal to V_(cc) (step 68). If V_(S) ≠V_(cc) (where V_(cc) is the circuitpower source voltage), a timer B (not shown in the drawings) in CPU 47is reset and counting up from zero by counter B is initiated (step 69).If on the other hand V_(S) =Vcc, then a decision is made as to whetheror not the count value T_(B) of timer B is greater than a time t₁ (step70). If T_(B) ≧t₁, then the value of pump current I_(P) is read in, anda decision is made as to whether or not I_(P) is smaller than a lowerlimit value I_(PLL) (step 71). If I_(P) <I_(PLL), then since a conditionof V_(S) =Vcc is continuously maintained while an excessive level ofpump current flow in the negative direction is occurring, this conditionis taken as indicating an open-circuit in the connecting leads ofelectrodes 17a and 17b of the sensor cell element 19, and therefore a"sensor cell element open-circuit" display command is issued to drivecircuit 46b (step 2). If I_(P) >I_(PLL), then a decision is made as towhether or not the feedback compensation coefficient K_(O2) (computed inthe K_(O2) computation subroutine) is smaller than the lower limit valueK_(O2LL) (step 73). If K_(O2) <K_(O2LL), then this shows that althoughan excessively high level of pump current does not flow, a condition ofV_(S) =Vcc is continuing and the feedback compensation coefficientK_(O2) is excessively small This is taken to indicate an open-circuit inthe connecting leads of electrodes 17a, 17b of sensor cell element 19,and a "sensor cell element open-circuit" display command is issued todrive circuit 46b (step 72).

After timer B has been reset, if it is judged that the count value T_(B)does not yet correspond to the time value t₁, or if it is judged thatK_(O2) ≧K_(O2LL), then a decision is made as to whether or not the pumpcurrent value I_(P) is 0 (mA) (step 74). If I_(P) ≠0 (mA), then a timerC (not shown in the drawings) in CPU 47 is reset and begins counting upfrom zero (step 75). If on the other hand I_(P) =0 (mA), then a decisionis made as to whether or not the count value T_(C) of timer C is greaterthan the time t₂ (step 76). If T_(C) ≧t₂, then a decision is made as towhether or not the feedback compensation coefficient K_(O2) is greaterthan the upper limit value K_(O2LH) (step 77). If K_(O2) >K_(O2LH), thenthis is taken to indicate an open-circuit in the connecting leads ofelectrodes 16a and 16b of oxygen pump element 18 while the air/fuelratio is more rich than the target air/fuel ratio, whereby a conditionof I_(P) =0 (mA) is being continued, so that the feedback compensationcoefficient K_(O2) is excessively high. An "oxygen pump elementopen-circuit" display command is therefore issued to drive circuit 46b(step 78). If K_(O2) ≦K_(O2LH), then a decision is made as to whether ornot the feedback compensation coefficient K_(O2) is smaller than thelower limit value K_(O2LL) (step 79). If K_(O2) ≦K_(O2LL), then this istaken to indicate that a condition of I_(P) =0 (mA) is continuouslyestablished while the feedback compensation coefficient K_(O2) isexcessively low due to an open-circuit in the connecting leads ofconnecting leads 16a, 16b of the oxygen pump element 18, so that theair/fuel ratio is more lean than the target air/fuel ratio. An "oxygenpump element open-circuit" display command is therefore issued to drivecircuit 46b (step 78).

After timer C has been reset, if it is judged that the count value T_(C)does not yet correspond to the time value t₂, or if it is judged thatK_(O2) ≧K_(O2LL), then a decision is made as to whether or not thefeedback compensation coefficient K_(O2) is greater than the upper limitvalue K_(O2LH) (step 80). If K_(O2) ≦K_(O2LH), then timers D and E(neither of which is shown in the drawings) in CPU 47 are respectivelyreset and begin counting up from zero (steps 81, 82). If on the otherhand K_(O2) >K_(O2LH), then a decision is made as to whether or not thecount value T_(D) of timer D is greater than a time interval t₃ (step83). If T_(E) <t₃, then timer D is reset and counting up by timer E fromzero is initiated (step 82). If T_(D) ≧t₃, then since it is unusual foran operating condition in which the air/fuel ratio is excessively leanto continue for more than time t₃, a decision is made as to whether ornot the voltage V_(S) between electrodes 17a and 17b of sensor cellelement 19 is lower than 0.4 (V) (step S4). If V_(S) <0.4 (V), then thisindicates that the air/fuel ratio is lean as a result of the value ofV_(S). Counter E is then reset and counting up by timer E from zero isinitiated (step 85). The engine speed of rotation N_(e) and the absolutepressure P_(BA) in the intake pipe are then read in, and a decision ismade as to whether or not the speed of rotation N_(e) is greater than3,000 (r.p.m.). In addition, a decision is made as to whether or not theabsolute pressure P_(BA) in the intake pipe is greater than 660 (mmH_(g)) (steps 86, S7), and a decision is made as to whether or not thetarget air/fuel ratio L_(ref) is set at a value lower than 14.7 (step88). The target air/fuel ratio may be determined in accordance withengine parameters representing engine load, such as N_(e) and P_(BA), byreading out a data map. If all of the predetermined conditions N_(e)≧3,000 (r.p.m.), P_(BA) ≧660 (mm H_(g)), and L_(ref) ≦14.7, are met,then this is taken to indicate that the engine is operating under acondition of high load with a rich air/fuel ratio. In this case, adecision is made as to whether or not the pump current I_(P) is higherthan the upper limit value I_(PLH) (step 89). If I_(P) >I_(PLH), thenthis shows that an excessively high level of pump current is flowing inthe positive direction, in spite of the fact that the air/fuel ratio isrich, and therefore this is taken to indicate that gas from the exhausthas leaked into the atmospheric reference chamber 15 due to a reasonsuch as a crack in the solid electrolytic member 11, so that the voltageV_(S) is lower than 0.4 (V). This is a "rich abnormality" detectioncondition, and a "rich abnormality detection" display command is issuedto drive circuit 46b (step 90). If at least one of the conditions N_(e)≧3,000 (r.p.m.), P_(BA) ≧660 (mm H_(g)), and L_(ref) ≧14.7 is notsatisfied, then a decision is made as to whether or not the pump currentI_(P) is greater than the lower limit I_(PLH) (step 91). If I_(P)>I_(PLH), then this indicates that although the feedback compensationcoefficient K_(O2) has been held continuously higher than the upperlimit value K_(O2LH), to compensate for a condition of lean air/fuelratio, and the air/fuel ratio has been thereby made rich, an excessivelyhigh level of pump current is flowing in the positive direction. This istaken to indicate, that a short-circuit exists between electrodes 16aand 16b of the oxygen pump element 18, and therefore an "oxygen pumpelement short-circuit" display command is issued to drive circuit 46b(step 92). The target air/fuel ratio L_(ref) is set in accordance withthe engine speed of rotation N_(e) and the absolute pressure P_(BA) inthe intake pipe, in synchronism with the TDC signal, during a subroutineof the fuel supply routine.

If it is judged in step 84 that V_(S) ≧0.4 (V), then this value of V_(S)is taken as indicating that the air/fuel ratio is rich. Therefore, sinceit is unusual for the feedback compensation coefficient K_(O2) to exceedthe upper limit value K_(O2LH), a decision is made as to whether or notthe count value T_(E) of timer E has exceeded a time interval t₄ (step93). If T_(E) >t₄, then this is taken to indicate that there is anopencircuit in the connecting leads of the heater element 20, or withinheater element 20 itself, and a "heater element open-circuit" displaycommand is issued to drive circuit 46b (step 94).

After resetting timer E in step 82 or if it is judged that the countvalue T_(E) has not yet attained time t₄, then a decision is made as towhether or not the feedback compensation coefficient K_(O2) is lowerthan the lower limit value K_(O2LL) (step 95). If K_(O2) ≧K_(O2LL), thena timer F (not shown in the drawings) within CPU 47 is reset, and beginscounting up from zero (step 96). If K_(O2) <K_(O2LL), then a decision ismade as to whether or not the count value T_(F) of timer F is greaterthan a time interval t₅ (step 97). If T_(F) ≧t₅, then a decision is madeas to whether or not the voltage V_(S) between electrodes 17a, 17b ofsensor cell element 19 is greater than 0.4 (V) (step 98). If V_(S) ≦0.4(V), then since such a value of V_(S) indicates that the air/fuel ratiois lean, and since it is unusual for the feedback compensationcoefficient K_(O2) to fall below the lower limit value K_(O2LL), thiscondition is taken to indicate that there is an open-circuit in theconnecting leads of heater element 20 or within heater element 20itself, and a "heater element open-circuit" display command is issued todrive circuit 46b (step 94). If on the other hand V_(S) >0.4 (V), then adecision is made as to whether or not the pump current I_(P) is smallerthan the lower limit value I_(PLL) (step 99). If I_(P) <I_(PLL), thenthis indicates that in spite of the fact that the feedback compensationcoefficient K_(O2) has been continuously held lower than the lower limitvalue K_(O2LL) to compensate for a condition of rich air/fuel ratio, sothat the air/fuel ratio has been made lean, an excessively high level ofpump current is flowing in the negative direction. This is taken toindicate that there is a short-circuit between electrodes 16a and 16b ofthe oxygen pump element 18, and therefore an "oxygen pump elementshort-circuit" display command is issued to drive circuit 46b (step 92).

If a "sensor cell element short-circuit" display command, a "sensor cellelement open-circuit" display command, or a "heater elementopen-circuit" display command has been issued, then the feedbackcompensation coefficient K_(O2) is made equal to 1, in order to haltair/fuel ratio feedback control operation (step 100). If an "oxygen pumpelement short-circuit" display command or an "oxygen pump elementopen-circuit" display command has been issued, then, operating under theassumption that the sensor cell element 19 is functioning normally,processing is executed to compute the feedback compensation coefficienton the basis of udgement of the air/fuel ratio in accordance with thevalue of voltage V_(S) developed between electrodes 17a and 17b ofsensor cell element 19 (step 101).

When drive circuit 46b is supplied with any of the display commandsdescribed above, a predetermined display pattern is produced upon adisplay unit 3S, in accordance with the contents of the command.

Each of the timers A through F can be implemented by supplying clockpulses to vary the contents of registers within CPU 47.

In the embodiment of the present invention described above, the fuelsupply quantity is controlled in accordance with a pump current valueI_(P). However, the present invention is not limited to such anarrangement, and it would be equally applicable to a method ofabnormality detection for an oxygen concentration sensor of an air/fuelratio control apparatus which employs a secondary air intake technique,whereby a quantity of secondary intake air could be controlled inaccordance with the pump current I_(P).

Furthermore in the embodiment of the present invention described above,the lead-in aperture 14 is utilized as a gas diffusion control means.However it is also possible to employ a porous material such as alumina(Al₂ O₃), formed as a porous body which fills a lead-in aperture or thegas holding chamber.

In this way, a method of abnormality detection for an oxygenconcentration sensor according to the present invention utilizes thefact that an air/fuel ratio which is judged on the basis of a voltagedeveloped between the electrodes of a sensor cell element and anair/fuel ratio which is judged on the basis of a value of pump currentwhich flows between the electrodes of an oxygen pump element. An engineoperating condition, or an air/fuel ratio compensation value (e.g.K_(O2)) that is derived in accordance with an oxygen concentrationsensing value and is used to control the air/fuel ratio of a mixturesupplied to the engine to maintain a target air/fuel ratio, will becomemutually opposite (i.e. with one air/fuel ratio being lean while theother is rich, or vice-versa). Thus, an abnormality of oxygenconcentration sensing can be reliably detected based upon the level ofvoltage developed between the electrodes of the sensor cell element andat least one of the following:

an engine operating condition;

the level of pump current which flows between the electrodes of theoxygen pump element;

or the air/fuel ratio compensation value which is utilized to controlthe air/fuel ratio of a mixture supplied to the engine to maintain thatratio at a target air/fuel ratio.

When an abnormality is detected by the method of the present invention,air/fuel ratio control in accordance with the output from the oxygenconcentration sensor can be immediately halted, whereby operation of theengine under a condition of reduced accuracy of control of the mixturesupplied to the engine can be prevented. Lowering of the effectivenessof engine exhaust gas pollution reduction can thereby be reliablyavoided.

What is claimed is:
 1. A method for controlling an air/fuel ratio of theintake air-fuel mixture of an internal combustion engine provided withan oxygen concentration sensor at the exhaust system thereof, saidsensor including a sensor unit having oxygen pump and sensor cellelements forming a gas diffusion central region therebetween into whichthe exhaust gas is supplied, voltage supply means for supplying a pumpvoltage determined in accordance with deviation of a sensor voltageappearing across said sensor cell element from a reference voltage so asto maintain said sensor voltage at said reference voltage, and signalgenerating means for generating an oxygen concentration signal varyingwith a pump current flowing through said oxygen pump element, saidmethod comprising:setting a target air/fuel ratio in accordance with theload of said engine; determining an air/fuel ratio control valve inaccordance with the engine operating conditions of said engine whilecomparing the oxygen concentration signal with said target air/fuelratio; controlling the intake air/fuel ratio in accordance with saidair/fuel ratio control value; detecting an appearance of a sensor valuenormally indicative of a rich condition in which the sensed intakeair/fuel ratio is withina rich region; comparing said sensor voltagewith said reference voltage; and determining an abnormality of saidoxygen concentration sensor according to when said sensor voltage islower than said reference voltage upon detection of the rich conditionin said step of detecting.
 2. The method as de fined by claim 1 whereinsaid step of detecting includes detecting a condition in which thesensed intake air/fuel ratio is smaller than an air/fuel ratiocorresponding to said reference voltage.
 3. The method as defined byclaim 1 wherein said step of detecting includes detecting that saidtarget air/fuel ratio is smaller than the stoicheometric air/fuel ratio.4. The method as defined by claim 1, in which said step of detectingincludes detecting that the engine rotational speed is higher than areference level.
 5. The method as defined by claim 1, wherein said stepof detecting includes detecting that the absolute pressure within anintake manifold downstream of a throttle valve is higher than areference level.
 6. The method as defiend by claim 1, wherein areference gas region sandwiches said sensor cell element together withsaid gas diffusion control region.
 7. A method for controlling anair/fuel ratio of the intake air/fuel mixture of an internal combustionengine provided with an oxygen concentration sensor at the exhaustsystem thereof, said sensor including a sensor unit having oxygen pumpand sensor cell elements forming a gas diffusion control regiontherebetween into which the exhaust gas is supplied, voltage supplymeans for supplying a pump voltage determined in accordance withdeviation of a sensor voltage appearing across said sensor cell elementfrom a reference voltage so as to maintain said sensor voltage at saidreference voltage and signal generating means for generating an oxygenconcentration signal varying with a pump current flowing through saidoxygen pump element, said method comprising:setting a target air/fuelratio in accordance with the load of said engine; determining anair/fuel ratio control value in accordance with the engine operatingconditions of said engine while comparing the oxygen concentrationsignal with said target air/fuel ratio; controlling the intake air/fuelratio in accordance with said air/fuel ratio control value; detecting anappearance of a sensor value normally indicative of a rich condition inwhich the sensed intake air/fuel ratio is within a rich region;comparing said pump current with a critical high level; and determiningan abnormality of said oxygen concentration sensor according to whensaid pump current is larger than said critical high level upon detectionof the rich condition in said step of detecting.
 8. The method asdefined by claim 7 wherein said step of detecting includes detecting acondition in which the sensed intake air/fuel ratio is smaller than anair/fuel ratio corresponding to said reference voltage.
 9. The method asdefined by claim 7 wherein said step of detecting includes detectingthat said target air/fuel ratio is smaller than the stoicheometricair/fuel ratio.
 10. The method as defined by claim 7, in which said stepof detecting includes detecting that the engine rotational speed ishigher than a reference level.
 11. The method as defined by claim 7,wherein said step of detecting includes detecting that the absolutepressure within an intake manifold downstream of a throttle valve ishigher than a reference level.
 12. The method as defined by claim 7,wherein a reference gas region sandwiches said sensor cell elementtogether with said gas diffusion control region.
 13. A method forcontrolling an air/fuel ratio of the intake air-fuel mixture of aninternal combustion engine provided with an oxygen concentration sensorat the exhaust system thereof, said sensor including a sensor unithaving oxygen pump and sensor cell elements forming a gas diffusioncontrol region therebetween into which the exhaust gas is supplied,voltage supply means for supplying a pump voltage determined inaccordance with deviation of a sensor voltage appearing across saidsensor cell element from a reference voltage so as to maintain saidsensor voltage at said reference voltage, and signal generating meansfor generating an oxygen concentration signal varying with a pumpcurrent flowing through said oxygen pump element, said methodcomprising:determining a basic air/fuel ratio control value inaccordance with the engine operating conditions o said engine; setting atarget air/fuel ratio in accordance with the load of said engine;deriving a compensation value which increases as long as an air/fuelratio represented by the oxygen concentration signal is leaner than saidtarget air/fuel ratio; correcting said basic air/fuel ratio controlvalue by said compensation value; controlling the intake air/fuel ratioin accordance with the corrected basic air/fuel ratio control value;comparing said sensor volatge with said reference voltage and saidcompensation value with a predetermined value; and determining anabnormality of said oxygen concentration sensor according to when saidsensor voltage is lower than said reference voltage and saidcompensation value is larger than said predetermined value.
 14. Themethod as defined by claim 13, wherein a reference gas region sandwichessaid sensor cell element together with said gas diffusion controlregion.
 15. A method for controlling an air/fuel ratio of an intakeair-fuel mixture of an internal combustion engine progided with anoxygen concentration sensor at the exhaust system thereof, said sensorincluding a sensor unit having oxygen pump and sensor cell elementsforming a gas diffusion control region therebetween into which theexhaust gas is supplied, and a reference gas region sandwiching saidsensor cell element together with said gas diffusion control region,voltage supply means for supplying a pump voltage determined inaccordance with deviation of a sensor voltage appearing across saidsensor cell element from a reference voltage so as to maintain saidsensor voltage at said reference voltage, and signal generating meansfor generating an oxygen concentration signal varying with a pumpcurrent flowing through said oxygen pump element, said methodcomprising:setting a target air/fuel ratio in accordance with the loadof said engine; determining an air/fuel ratio control value inaccordance with the engine operating conditions of said engine whilecomparing the oxygen concentration signal with said target air/fuelratio; controlling the intake air/fuel ratio in accordance with saidair/fuel ratio control value; comparing said pump current with acritical high level and said sensor voltage with said reference voltage;and determining an abnormality of said oxygen concentration sensoraccording to when said pump current is larger than said critical highlevel and said sensor voltage is lower than said reference voltage.